1. Field of the Invention
The present invention relates to an optical scanner in which a rotating polygon mirror is used as a deflector, and more particularly to fluctuation in spot shapes on a scan surface.
2. Description of the Related Art
Optical scanners are well known in which optical scanning is performed by a light beam, deflected and reflected by a rotating polygon mirror used as a deflector. This type of optical scanner is utilized in recorders or readers where a laser light, etc., is used as a light source source, such as printers, scanners, radiation image information readers and the like. The optical scanner is generally constructed of a light source for emitting a light beam, a collimator lens for collimating the light beam emitted from the light source, a first image-forming optics system, and a second image-forming optics system. The first image-forming optics system consists of a cylindrical lens, etc., disposed so that the light beam from the collimator lens is formed near the reflection surface (deflection surface) of a rotating polygon mirror as the line image. The second image-forming optics system consists of the rotating polygon mirror for deflecting and reflecting the light beam emitted from the first image-forming optics system, an fxcex8 lens and a cylindrical lens for converging a beam Q, deflected and reflected by the rotating polygon mirror 20, at a predetermined scan surface, and for scanning the beam Q on the scan surface at uniform velocity, and the like.
It is known that such an optical scanner adopts an oblique incidence optics system (e.g., Japanese Unexamined Patent Publication No. 62(1987)-30214). In this optical scanner, when it is assumed that a plane always including a light beam, deflected and reflected by the rotating polygon mirror, is a horizontal scanning plane and that a plane, perpendicular to the horizontal scanning plane, which includes the central axis (optical axis) of the second image-forming optics system, is a vertical scanning plane, a light beam is incident and deflected at an angle to the horizontal scanning plane. In the oblique incidence optics systems, including the one disclosed in the above-mentioned publication, the angle of a light beam incident on the rotating polygon mirror with respect to the vertical scanning plane, i.e., the angle of incidence in the horizontal scanning direction, is normally considered to be 0 degrees.
As described above, in the case where a light beam is caused to be incident on the rotating polygon mirror, with the angle of incidence in the horizontal scanning direction being 0 degrees, there is a need to increase the angle of the light beam incident on the rotating polygon mirror with respect to the horizontal scanning plane (i.e., the angle of incidence in the vertical scanning direction) to a certain degree, to avoid mechanical interference between the first image-forming optics system and the second image-forming optics system. On the other hand, if the angle of incidence in the vertical scanning direction is made larger, there is a problem that the curvature of the scanning line on a scan surface will be increased.
Hence, for example, it is considered that the above-mentioned mechanical interference can be avoided by causing the light beam to be incident on the rotating polygon mirror at an angle to the horizontal scanning direction as well as to the vertical scanning direction, and that the curvature of the scanning line can be suppressed by making the angle of incidence in the vertical scanning direction smaller.
On the other hand, in the above-mentioned optical scanner, the parallelism between the reflection surface of the rotating polygon mirror and the axis of rotation of the rotating polygon mirror normally varies from reflection surface to reflection surface. To prevent the occurrence of scanning-line position fluctuation (pitch fluctuation) in which the position of a scanning line on the scan surface fluctuates in the vertical scanning direction substantially perpendicular to the direction of the scanning line because of the aforementioned variation in the parallelism, the aforementioned two cylindrical lenses are provided on the entrance side and exit side of the rotating polygon mirror and constitute a parallelism correction optics system for correcting for the aforementioned scanning-line position fluctuation (pitch fluctuation).
In the case where the parallelism correction optics system is provided, as described above, the line image of an incident beam is formed on the reflection surface of the rotating polygon mirror. In this case, if a light beam is obliquely incident in the vertical scanning direction with the angle of incidence in the horizontal direction being 0 degrees, the position of a line image on the reflection surface of the rotating polygon mirror fluctuates according to the scanning angle, and the gradient of the line image becomes symmetrical with respect to a scanning angle of 0 degrees. The shape of a spot on the scan surface becomes a nearly true circle at a scanning angle of 0 degrees. When the scanning angle is xcex8, the spot shapes both become a longitudinally long and small ellipse and also become symmetrical with each other.
On the other hand, if a light beam is obliquely incident not only in the vertical scanning direction but also in the horizontal scanning direction, the gradient of a line image on the reflection surface becomes asymmetrical. Because of this, for example, if the shape of a spot on the scan surface has a nearly true circle at a scanning angle of xe2x88x92xcex8, it becomes a longitudinally long and large ellipse when the scanning angle is +xcex8.
The present invention has been made in view of the above-mentioned drawbacks found in the prior art. Accordingly, it is the primary object of the present invention to provide an optical scanner which is capable of suppressing fluctuation in spot shapes on a scan surface, i.e., maintaining the symmetry of spot shapes, even when an oblique incidence optics system for causing a light beam to be incident at an angle to the horizontal scanning direction as well as to the vertical scanning direction is adopted to suppress the curvature of a scanning line.
To achieve this end, there is provided an optical scanner comprising:
a collimator lens for collimating a light beam emitted from a laser light source;
a rotating polygon mirror for deflecting and reflecting the light beam;
a cylindrical lens disposed so that the light beam from the collimator lens is formed near a reflection surface of the rotating polygon mirror as a line image; and
an image-forming optics system for converging the light beam, deflected and reflected by the rotating polygon mirror after having passed through the cylindrical lens, at a predetermined scan surface, and for scanning the converged light beam on the surface at uniform velocity;
the light beam being obliquely incident on the reflection surface with respect to both a first plane perpendicular to an axis of rotation of the rotating polygon mirror and a second plane, perpendicular to the first plane, which includes the central axis of the image-forming optics system;
wherein the cylindrical lens is disposed so that its generating line is substantially parallel with the first plane.
The words xe2x80x9cfirst planexe2x80x9d refer to a plane (deflection plane) that the normal of the reflection surface forms by rotation of the rotating polygon mirror.
In a preferred form of the present invention, when it is assumed that a plane, including both the central axis (z) of the image-forming optics system and an x-axis (x) perpendicular to both the central axis (z) and the axis of rotation of the rotating polygon mirror, before the rotating polygon mirror is inclined, is an x-z plane and that a plane, including both a y-axis (y) perpendicular to the x-z plane and the central axis (z), is a y-z plane, an angle xcex1, which is formed within the x-z plane by both the light beam incident on the reflection surface of the rotating polygon mirror and the central axis (z), an angle xcex2, which is formed by the y-z plane and the light beam incident on the reflection surface, an angle xcex3, which is formed by the generating line of the cylindrical lens and the x-z plane, an angle xcfx86, which is formed by both the axis of rotation projected on the x-y plane and the central axis (z), and an angle xcex4, which is formed by the y-axis (y) and the axis of rotation, satisfy the following equation:
xcex3=tanxe2x88x921{(cos xcex1 sin xcfx86 sin xcex4xe2x88x92sin xcex1 cos xcfx86 sin xcex4)/(sin xcex1 sin xcex2 sin xcfx86 sin xcex4+cos xcex2 cos xcex4+cos xcex1 sin xcex2 cos xcfx86 sin xcex4)}xe2x80x83xe2x80x83(1)
According to the optical scanner of the present invention, the cylindrical lens is disposed so that its generating line is substantially parallel with the first plane perpendicular to the axis of rotation of the rotating polygon mirror. As a result, the gradient of a line image formed on the reflection surface of the rotating polygon mirror becomes approximately symmetrical with a scanning angle of 0 degrees as reference, and fluctuation in spot shapes on a scan surface can be suppressed.
In addition, if an angle xcex3 which is formed by both the generating line of the cylindrical lens and the x-z plane is set to an angle prescribed by the above-mentioned Eq. (1), the generating line of the cylindrical lens can be made substantially parallel with the first plane perpendicular to the axis of rotation of the rotating polygon mirror.